1. Field of the Invention
The present invention relates to an organic light emitting device formed by organic light emitting elements such as organic electroluminescence elements having emission layers
2. Description of the Prior Art
In recent years, an organic electroluminescence element (hereinafter referred to as an organic EL element) having excellent characteristics such as a wide viewing angle, high-speed responsibility, low power consumption and the like is energetically studied. The basic structure of the organic EL element is obtained by forming an organic thin film containing a luminescent material between a transparent electrode (hole injection electrode) of ITO (indium-tin oxide) or the like and a cathode (electron injection electrode) of a material having a small work function. This organic EL element emits light due to recombination of holes and electrons, injected from the transparent electrode and the cathode respectively, in the organic thin film containing the luminescent material (refer to C. W. Tang and S. A. Van Slyke, Applied Physics Letters, Vol. 51, No. 12, pp. 913 to 915, 1987).
In an organic light emitting device employing such organic EL elements, a plurality of data electrodes (hole injection electrodes) of transparent conductive films are arranged on a glass substrate in the form of stripes, and a hole transport layer, an emission layer and an electron transport layer are stacked on the data electrodes, while a plurality of scan electrodes are arranged on the electron transport layer to be perpendicular to the data electrodes. Thus, organic EL elements are formed on the intersections between the plurality of data electrodes and the plurality of scan electrodes, for forming a dot matrix of the plurality of organic EL elements.
Methods of driving such an organic light emitting device formed by a dot matrix of a plurality of organic EL elements can be roughly classified into two systems, i.e., a passive matrix driving system and an active matrix driving system. In the passive matrix driving system, organic EL elements arranged on intersections between a plurality of scan electrodes and a plurality of data electrodes are driven in a time-sharing manner. In the active matrix driving system, organic EL elements are provided on intersections between a plurality of scan electrodes and a plurality of data electrodes through switching elements, to be selectively driven by the switching elements.
FIG. 7 is a schematic plan view showing a conventional organic light emitting device of the passive matrix driving system employing organic EL elements. FIG. 8 is a sectional view of the organic light emitting device taken along the line D—D in FIG. 7.
As shown in FIGS. 7 and 8, a plurality of striped data electrodes 2 vertically extending along arrow Y are arranged on a transparent substrate 1 of glass. Each of FIGS. 7 and 8 illustrates only three data electrodes 2. The data electrodes 2 are formed by transparent conductive films of ITO (indium-tin oxide) or the like. Such data electrodes 2 have high electric resistance, and hence vertically extending bus lines 3 are formed on partial regions of the data electrodes 2 or adjacently in contact with the data electrodes 2, in order to ensure conductivity. The bus lines 3 are formed by low-resistance metal films of Cr/Mo/Cr or the like.
An organic thin film 6 including a hole transport layer, an emission layer and an electron transport layer is formed on the data electrodes 2. A plurality of striped scan electrodes 7 horizontally extending along arrow X are arranged on the organic thin film 6 to be perpendicular to the data electrodes 2. Organic EL elements are formed on intersections where the data electrodes 2 and the scan electrodes 7 oppositely intersect with each other. Each organic EL element forms a single pixel. Barrier layers 8 of a photoresist material are provided between the plurality of scan electrodes 7. Thus, the plurality of scan electrodes 7 are isolated from each other.
The aforementioned organic light emitting device of the passive matrix driving system can advantageously be more readily manufactured at a lower cost as compared with an organic light emitting device of the active matrix driving system having a plurality of switching elements arranged on a substrate.
In order to drive the organic light emitting device of the passive matrix driving system, a voltage is successively applied to the plurality of scan electrodes 7 in one frame. Thus, a row of pixels located under each scan electrode 7 are selected so that each pixel enters a luminous state or a non-luminous state in response to a voltage applied to the data electrodes 2.
The organic light emitting device of the passive matrix driving system is desired to be improved in luminance and definition and increased in size, to be capable of displaying continuous motion pictures.
In order to improve the definition and increase the size of the organic light emitting device, the number of the scan electrodes 7 must be increased. When the number of the scan electrodes 7 is increased, however, the number of the rows of the pixels successively selected in one frame is so increased that the selection time for each pixel is reduced to reduce the duty ratio. The term “duty ratio” stands for the ratio of the time when each pixel is selected in one frame. When the duty ratio is reduced, luminance visually recognized by human eyes is reduced.
In order to ensure sufficient luminance in the organic light emitting device, the selected pixels must emit light in high luminance. Therefore, the organic EL element forming each pixel must be driven at a high voltage. In this case, a high electric field is applied to each organic EL element, to increase the temperature. When an organic EL element is left under a high electric field and a high temperature in general, deterioration of the organic material rapidly progresses to remarkably reduce reliability of the element. Therefore, it is difficult to improve the luminance and definition and increase the size of the organic light emitting device while ensuring the reliability.
As hereinabove described, a technique of providing the barrier layers 8 between the scan electrodes 7 by patterning a photoresist material is employed for isolating the plurality of scan electrodes 7 from each other. However, the aperture ratio (the ratio of the pixel region to a display region) of the pixels is remarkably reduced due to the barrier layers 8 inserted between the scan electrodes 7. In order to compensate for such reduction of the aperture ratio of the pixels, each pixel must emit light in high luminance. Thus, the reliability of the element is remarkably reduced as described above.
When the barrier layers 8 of a photoresist material are formed between the plurality of scan electrodes 7, the photoresist material must be patterned into an optimum shape. However, the photoresist material is generally patterned through a wet process, and hence it is difficult to pattern the photoresist material into a precise shape due to residues etc. resulting from the patterning. Consequently, the non-defective ratio of the organic light emitting device is reduced.
Further, the barrier layers 8 of a photoresist material contain a larger amount of moisture as compared with the data electrodes 2, the organic thin film 6 and the scan electrodes 7. This moisture may gradually permeate into the scan electrodes 7, to oxidize a metal. Thus, current injection efficiency is extremely reduced, to result in dark spots. Or, the moisture permeating from the barrier layers 8 may inactivate the interface between the organic thin film 6 and the scan electrodes 7. Also in this case, light emitting potions are gradually contracted, to result in dark spots. Such permeation of the moisture into the interface between the organic thin film 6 and the scan electrodes 7 progresses from both ends of the scan electrodes 7 isolated from each other by the barrier layers 8. Thus, the number of portions invaded by the moisture is increased as the number of the scan electrodes 7 is increased, to accelerate deterioration of element characteristics.